Mutualism, Mice and Mallards: Relationships of Ground Nesting
Birds in the Uplands of California

Searches of upland habitat have revealed high densities and success of duck
nests near major wetland regions of California. Northern harriers and short-eared
owls were also found frequently nesting in the same habitats as were ducks.
These species showed high variability in densities among years and geographic
location. Previous studies have demonstrated that abundance of mice is extremely
important for stimulating nesting in these species. In fields where nesting
densities and success of harriers and owls were high, nest success of mallards
exceeded 80% (Mayfield). Cinnamon teal, another nester in California uplands,
did not nest where harriers were abundant. Northern harriers and to a lesser
extent, short-eared owls are very aggressive defenders of territories around
their nest sites. This trait may provide protection for mallards nesting close
to owls and harriers. Northern harriers are effective predators of ducklings
and have been observed feeding on young mallards that were more than half the
size of adults (Class IIc). High nest success of mallards near harriers and
owls would likely offset potential duckling losses, however. Adult cinnamon
teal may choose to forego nesting next to harriers because they may be too small
to deter attack. These observations suggest that predator-prey relationships
may be complex and must be carefully considered before broad-scale imposition
of predator control.
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